Dependable float valves are normally supplied complete with float from a number of suppliers.
The art is to have the correct fitting accuracy and material for seat and seat sealings in the valve (as for all valves), and the correct arm and float shape and geometrical configuration to operate the valve properly ,also with correct materials.
For diesel fuel the most common float material (for all purposes) would be a standard supplied SS float.
For a closed cell foam float combined with diesel I would presume a closed protecting surface of some mm thickness would be necessary, and of course in a material longtime compatibel with diesel, and also compatibel with (sea?)water and other possible contaminations in a diesel tank, including detergents or heat (steam) if this is applicable for your tank(s) for longtime life services.
A suggestion would be an existing commercial available suitable hollow form or ball of some plastic (or metallic) material(tubes or boxes with lids)and then fill this up with your foam and extra-seal the openings and attachment to the arm.
.. but I dont see the necessity or market for this construction, as I believe the market is fairly well covered with standard valves reasonably priced with air-filled floats in SS.
If you only need a small float for one valve you could probably get the cheapest and highest-quality float from the spare float for a steam-trap with float in SS or SS316, or as a sparefloat from a standard float-vale supplier.
